1. Field of the Invention
The present invention relates to a circular accelerator for circulating a charged-particle beam and extracting the beam and a method and an apparatus for extracting the charged-particle beam.
2. Description of Related Art
Conventionally, a circular accelerator is arranged to circulate a charged-particle beam containing accelerated electrons or ions and extract the beam out of the circulating orbit. A transport line is used to transport the extracted beam to a location where it is used for physical experiment or medical use. For the conventional method for extracting the charged-particle beam, the resonance of betatron oscillation caused in the beam has been utilized as discussed in AIP Conference Proceedings No. 127 (1983), pages 53 to 61.
The resonance of the betatron oscillation is a phenomenon as follows. The charged particles circulate while oscillating right and left or up and down. This is referred to as a betatron oscillation. The number of betatron oscillations per one circulation is referred to as a tune. The tune can be controlled by a bending electromagnet or a four-pole electromagnet. When a resonance-generating six-pole electro magnet provided in a circulating orbit is excited at a time when the tune comes closer to an integer .+-.1/3, an abrupt increase of a betatron oscillation amplitude takes place for those charged particles, which have higher betatron oscillation amplitudes than a given threshold value, among the circulated charged particles. This phenomenon is referred to as a resonance of betatron oscillation. The threshold value is referred to as a stability limit. The magnitude of the betatron oscillation amplitude of the stability limit varies depending on a deviation of the tune from an integer .+-.1/3. It becomes smaller as the tune comes closer to an integer .+-.1/3. By utilizing this characteristic, in the conventional technique, the tune is gradually approached to an integer .+-.1/3, that is, the stability limit is gradually made smaller from an initial large value, so that the resonance first takes place in the charged particles having larger betatron oscillation amplitudes among the circulated charged particles and then the occurrence of the resonance is gradually prevailed to the charged particles having smaller betatron oscillation amplitudes, thereby beam extracting gradually the charged-particle beams.
As another method for extracting a charged-particle beam, a kicker electromagnet has been used as discussed in "Design of Synchrotron for Injection" UVSOR-7 (March, 1981), Particle Science Laboratory, pages 26 to 27 and 81 to 87.
The foregoing related arts have the following problems.
At first, it has a problem such that if the stability limit becomes smaller, the beam collides against a deflector wall provided at an extracting port, so that the charged particles may not be extracted. That is, even though the betatron oscillation amplitudes of the charged particles are substantially uniformly distributed, it is difficult to extract out the charged particles having betatron oscillation amplitudes lower than a certain value. This results in lowering an efficiency in extraction of the charged particles.
Second, it has another problem such that the orbit gradient of the charged particles extracted at the stability limit changes at the extracting port. Since the extracting deflector is located at a fixed angle with the circulating orbit, the charged particles, which are extracted at an angle deviated from the fixed angle by more than a certain angle, may collide with an inner wall of the transportation system including the extracting deflector to disappear. It lowers an efficiency in extraction of the charged particles. As another shortcoming, the extraction current changes and it is difficult to control it at a desired state. When the orbit gradient of the charged particles changes, the position at the outlet of the transportation system where the charged particles are extracted is also changed.
Third, it has a further problem such that the increment of the betatron oscillation amplitude per one circulation changes as the beam is being extracted, resulting in variation of the beam diameter.
Fourth, it has a still further problem such that the change of the extracting position at the outlet of the transportation system or the change of the extracting current or the beam diameter as the beam is being extracted is not preferable to any physical experiment or medical treatment.
Fifth, it has still another problem such that when the excitation of the four-pole electromagnet is changed for reducing the stability limit, the stability limit temporarily disappears and then again restores. No resonance takes place in a part of the beams, resulting in decrease of an extraction efficiency.
Sixth, it has a still further problem such that in order to obtain the sufficiently large strength of a magnetic field to extract the beam, many kicker electromagnets are needed. This prevents reducing of the accelerator size.